1. Field of the Invention
The present invention pertains to systems for scanning surface patterns of semiconductor wafers or the like, and more particularly, it pertains to imaging systems utilizing high resolution microscopes with computer controlled scanning devices to provide detailed and accurate output information for detecting defects or for providing accurate spatial measurments.
2. Description of the Prior Art
In the scanning of semiconductor wafers or the like to detect surface pattern defects, a variety of techniques have been utilized that take advantage of various forms of microscopes, both optical and acoustical, having high degrees of image resolution. In optical imaging systems generally, devices similar to T.V. cameras have been utilized wherein electromagnetic radiation is reflected from a relatively large spot on the wafer and processed through an optical system and photodetector device to provide a multi-intensity image which, either digitally or by analog means, can be recreated on an appropriate output device, such as a CRT.
More recently microscope systems for rapidly scanning specimens at high resolution have become available. One such system utilizes an acoustic microscope with the acoustic lens being mounted on a leaf spring arrangement permitting an electromagnetic vibratory drive to rapidly move the lens in a planar mode at a 50 Hz line scan frequency. The specimen is mounted on x-y tilt stages and is provided with separate x-y slow drives to bring the desired portion of the specimen into the field of view of the acoustic lens. A z-axis drive comprised of a piezoelectric stage drive is also provided on the specimen mounting to adapt the microscope for viewing different surface levels on the specimen.
Another recent semiconductor wafer scanning system utilizes a scanning optical microscope with a confocal imaging system. An image is built up on a T.V.-type display by mechanically scanning the wafer across a focused laser spot.
In scanning devices which use a confocal optical arrangement, sharply focused beams, as for example from a laser source, can be directed through the lens system and concentrated on a very small spot on the specimen, then reflected back through the lens system to a detector. By using this type of optic and by scanning the specimen rapidly in a planar mode, together with suitable electronics connected to the photodetector device, an image can be reconstructed based on the reflected intensity from the small spot as it scans over a segment of the specimen in a series of passes. A scanning microscope utilizing a confocal optical system is shown in U.S. Pat. No. 4,198,571 to Sheppard.